A foot may experience significant forces during walking, running, and other ambulatory activities. During running, for example, the motion of the foot generally proceeds as follows: Initially, a heel region of the foot strikes the ground, followed sequentially by an arch region and a forefoot region of the foot. As the heel region leaves the ground, the foot rolls forward so that the forefoot region, including the toes, makes contact with the ground. Finally, the entire foot leaves the ground to begin another cycle. During the time that the foot is in contact with the ground, the foot typically rolls from the outside (i.e., lateral side) to the inside (i.e., medial side), a process called pronation. That is, normally, the outside of the heel region strikes the ground first, and the toes on the inside of the foot leave the ground last.
Supporting the weight of the individual, imparting balance to the individual, and flexing to accommodate the motion of the foot discussed above may induce bending, tensile, and compressive forces in the various bones, muscles, ligaments, and tendons associated with the foot. Compressing a plantar surface (i.e., lower surface) of the foot against the ground or the interior of an article of footwear also induces compressive forces in the plantar surface. A magnitude of the compressive forces experienced by the plantar surface of the foot varies throughout the plantar surface. That is, some areas of the plantar surface experience forces with a relatively high magnitude, while other areas of the plantar surface experience forces with a relatively low magnitude.
An article of athletic footwear generally incorporates a sole structure with a polymer foam material that attenuates forces experienced by the foot. Fluid-filled bladders, for example, may be encapsulated within the polymer foam material to further assist with force attenuation, and moderators may be incorporated into the sole structure to distribute forces with a higher magnitude over a greater area of the plantar surface, thereby decreasing the peak forces experienced by an area of the plantar surface. Despite these footwear elements, however, the compressive forces experienced by the foot continue to vary throughout the plantar surface.